Book Two — Creation, Chapter 89 — Summa Contra Gentiles
[1] In order to facilitate the solution of these arguments, certain things must be premised in explanation of the order and process of the generation of man and of animals in general.
[2] To be taken into account first of all is the falsity of the opinion of those who say that the vital operations appearing in the embryo before its complete development do not proceed from a soul, or from a soul’s power existing in the embryo, but from the soul of the mother. If this were true, the embryo would not even be an animal, since every animal consists of soul and body. Vital operations, moreover, do not issue from an extrinsic active principle, but from an internal power; and in this respect particularly are living things, to which self-movement properly belongs, seen to differ from the non-living. For the thing that is nourished assimilates the nourishment and thus must possess an active power of nutrition; what the agent effects is like to itself. And this fact is much more manifest in the operation of the senses; it is through a power existing in this person, and not in another, that he is enabled to see and to hear. Hence, nourishment and even sensation on the part of the embryo prior to its complete development cannot be attributed to the soul of the mother.
[3] Nevertheless, it cannot be said that the soul in its complete essence is present in the semen from the very beginning, though its operations are not manifested because of the lack of organs. This is impossible in view of the fact that since the soul is united to the body as its form, it is united only to a body of which it is properly the act. Now, a soul is the “act of an organic body.” Prior to the organization of the body, therefore, the soul is not in the semen actually, but only potentially or virtually. Thus, Aristotle says in De anima II [1] that “seeds and fruits are endowed with life potentially so far as they are rid of,” that is, lack, “a soul; whereas the thing of which the soul is the act has indeed the power of life, but is not without a soul.”
[4] And the hypothesis of the soul’s presence in the semen from the beginning would entail the further consequence that animal generation takes place solely by way of partition, as with annulose animals, where two are produced from one. For, if the semen were possessed of a soul at the moment of its separation, it would then already be endowed with a substantial form. But in every case substantial generation precedes the substantial form; it never comes after it; and if any changes follow in the wake of the substantial form, they concern not the being but the well-being of the thing generated. Thus, the engendering of the animal would be completed with the mere alienation of the semen; and all subsequent changes would have no bearing upon the process of generation.
[5] But this theory would be even more ridiculous if applied to the rational soul. For, first, the soul cannot possibly be divided as the body is, so as to be present in the separated semen; and second, it would follow that in all extra-copulative emissions of semen, without conception taking place, rational souls would nevertheless be multiplied.
[6] Another theory, likewise inadmissible, is stated as follows. From the moment of severance the soul is not present in the semen actually but virtually, because of the lack of organs and yet this very power of the semen—itself a body potentially endowed with organs though actually without them—is, proportionately to the semen, a potential but not an actual soul. Moreover, since plant life requires fewer organs than animal life, from the moment that the organic development of the semen suffices for plant life, the aforesaid seminal power becomes a vegetative soul; and later, the organs having been perfected and multiplied still more, the same power is raised to the level of a sensitive soul; and finally, with the perfecting of the organs form, the same soul becomes rational, not indeed, by the action of that seminal power, but through the influx of an external agent. And for this reason the proponents of the theory suppose Aristotle to have said in the De generatione animalium that “the intellect is from without” [II, 3]. Now, this theory would involve the consequence that numerically one and the same power is at one time a purely vegetative soul, and afterwards a sensitive soul, the substantial form itself thus being perfected successively more and more. It would further follow both that the substantial form would be brought from potentiality to act, not all at once but in successive stages, and that generation is a continuous movement, just as alteration is. Now, all these consequences are impossible in nature.
[7] But that theory would entail a consequence still more incongruous, namely, the mortality of the rational soul. For nothing formal in character that accrues to a corruptible thing makes it incorruptible by nature; in that case, the corruptible would be changed into the incorruptible, which is impossible, since they differ in genus, as Aristotle says in Metaphysics X [10]. In the process described above, however, the substance of the sensitive soul is held to be generated accidentally by the generated body, and hence that substance must necessarily be corruptible with the corruption of the body. Therefore, if the same soul becomes rational through the infusion into it of a kind of light, having the role of a form in its regard, for the sensitive is potentially intellective, then necessarily the rational soul perishes along with the body. But this is impossible, as we proved above, and as the Catholic faith teaches.